Accepted, the Tutsi/Hutu division isn't one where difference in genetics/appearance seems to be a major factor, though I'd still assume the average Tutsi or Hutu could easily distinguish one from the other in a way outsiders mightn't be able to.
The main division of Tutsi/Hutu was primarily done by Europeans, and the criteria was based on "those who owned cattle became known as the Tutsi and those who did not own cattle became known as the Hutu", and taller persons were also assigned as Tutsi.
Taller men tend to earn more money on average so in those terms both the average Tutsi, Hutu and outsiders should be able to make a better than random guess about who is Tutsi or Hutu.
Ah you have it backwards - there was already the Tutsi ethnic group, but the Belgians found it easier to identify them as Tutsi by number of cattle etc.
"Prior to the arrival of colonists, Rwanda had been ruled by a Tutsi-dominated monarchy since the 15th century."
"Rwanda was ruled as a colony by Germany (from 1897 to 1916) and by Belgium (from 1922 to 1961). Both the Tutsi and Hutu had been the traditional governing elite, but both colonial powers allowed only the Tutsi to be educated and to participate in the colonial government. Such discriminatory policies engendered resentment."
"When the Belgians took over, they believed it could be better governed if they continued to identify the different populations. In the 1920s, they required people to identify with a particular ethnic group and classified them accordingly in censuses."
